1. Field of the Invention
This invention generally relates to mobile X-ray diagnostic systems, and in particular, relates to devices and methods for accurately determining the position and orientation of the system.
2. Description of the Related Art
Travel stands and support carriages for mobile X-ray diagnostic machines are known in the art. U.S. Pat. No. 6,374,937 discloses a mobile C-arm X-ray machine having a motorized carriage, which enables the machine to be remotely maneuvered to travel in a path essentially perpendicular to the plane of the C-arm in order to take a series of X-ray images about the body of the patient. A measurement system can be used to detect the path traveled by the motorized carriage. The Applicant's German Patent No. 4423359B4 discloses a mobile surgical X-ray diagnostic unit having a travel stand equipped with motor-operated rollers. The machine is outfitted with incremental transmitters, enabling a motorized movement of the C-arm along the floor. A position detector is used to detect the relative position of the travel stand along a straight path.
Users of present-day X-ray diagnostic machines often desire to use mobile X-ray diagnostic machines along with instrument navigation systems to support computer-aided surgery operations (CAS) or catheter diagnostics and therapy functions. In these types of systems, a three-dimensional (3D) volumetric model can be reconstructed from a number of two-dimensional (2D) X-ray projection images. An instrument, such as a catheter for example, can be navigated through certain parts of the patient's body while using the 3D volumetric model for guidance. The quality of the 3D reconstructed model often depends on precise knowledge of the projection geometry in each 2D projection image. Due to the lighter design of most mobile X-ray diagnostic machines as compared to stationary models, the actual X-ray projection geometries often deviate from the values theoretically calculated from kinematics. In order to remedy this drawback, these deviations are typically determined in a calibration run and placed in correction tables. This built-in calibration adjustment provides substantial improvement in the quality of the 3D reconstruction when the texture of the floor underneath the travel stand during calibration, which usually takes place at the factory, are virtually identical to the texture of the floor in an operating room where the medical procedure takes place. But this is often not the case and thus additional deviations arise, which usually cannot be determined in a calibration run. The deviations may be produced, for example, by the pressing of the rollers of the travel stand into an elastic floor covering, by a temporary or permanent flattening of the running surfaces of the rollers, or also by the spatially different elasticity of the floor, which occurs, for example, with hollow floors or floors with partial supports.
Partial differences in the elasticity and/or in the inclination of the floor also pose a problem and affect the accuracy of the system when, for example, it is necessary to track the path of a catheter tip in a subject and the path being observed has a greater length than the largest dimension of the X-ray receiver. In this case, an X-ray projection of the path must be put together from several smaller projection images, and the X-ray diagnostic machine usually has to move across the floor to obtain these different images. The same is true for the case when one has to create a volume model of a rather large and lengthy region of investigation from 2D projection images.
A mobile X-ray diagnostic machine can be used in conjunction with a navigational system for instrument navigation in a reconstructed 3D X-ray volume if the position of the reconstructed X-ray volume is known in relation to the system of coordinates of the navigational system. For this, the means of measurement provided in prior art systems are generally inadequate, since deviations of the floor from the horizontal and/or local fluctuations in the elasticity often cause false readings of position determination of the reconstructed X-ray volume.
The present invention is intended to ameliorate at least one of the drawbacks described above. To this end, certain embodiments of the present invention provide a mobile X-ray diagnostic machine which enables a determination of the orientation of the travel stand or cart in a spatially fixed system of coordinates even when the floor is uneven and/or has local differences in its elasticity.